This invention relates to the manufacture of a crystalline aluminosilicate material, and more particularly to the manufacture of low silicon faujasite-type zeolites. Specifically, the invention is directed to the preparation of low silicon sodium X zeolite having a very low concentration of by-product sodium A zeolite.
Faujasite-type zeolites are defined as those zeolites with a framework topography resembling the mineral faujasite. Such species are characterized by a relatively open zeolite framework with comparatively large (about 8 xc3x85) micropores and high (nearly 50%) intracrystalline void volumes. Synthetic faujasites are generally subdivided into zeolites X and Y by virtue of their silicon content; zeolite Y being characterized as synthetic faujasite having a Si/Al (silicon/aluminum) atomic ratio of 1.5 and higher, and zeolite X being characterized as synthetic zeolite having a Si/Al atomic ratio  less than 1.5. Type X itself can be further subdivided into low silicon type X (LSX), defined as type X zeolite having a Si/Al atomic ratio of 1.0 to about 1.1, medium silicon type X (MSX), defined as type X zeolite having a Si/Al atomic ratio in the range of  greater than 1.1 to about 1.2, and conventional to high silicon type X, defined as type X zeolite having a Si/Al atomic ratio in the range of  greater than 1.2 to about 1.5.
Conventionally, sodium zeolite X having Si/Al atomic ratios greater than about 1.2 are prepared by crystallizing a sodium-based aqueous gel at temperatures in the range of about 70 to about 100xc2x0 C. for several hours. In pioneering work in this field, Milton, in U.S. Pat. Nos. 2,882,243 and 2,882,244, described the preparation of type A and type X zeolites, and the use of these materials to separate gas mixtures such as air into their components.
Many other workers in the field have patented techniques for the preparation of zeolites. For example, Robertson, U.S. Pat. No. 4,173,622, and Strack et al., U.S. Pat. No. 4,303,629, describe the production of type A zeolite, and Kostinko, U.S. Pat. No.4,443,422, describes the preparation of zeolite A and zeolite X . Kostinko gives a detailed summary of the patent literature in the field of zeolite preparation.
Recently, there has been considerable interest in producing type X zeolites having low Si/Al atomic ratios. Attempts were made to prepare sodium LSX (NaLSX) zeolite by direct synthesis of sodium ion-containing gels; however, the resulting products contained considerable amounts of type A zeolite, which render them unsuitable for many adsorption applications. Tatic, M. et al. in xe2x80x9cZeolites: Synthesis, Structure, Technology and Applicationxe2x80x9d, Studies in Surface Science and Catalysis, vol. 24, pp.129-136 (1985), describe various methods to directly synthesize NaLSX. In most cases, a significant amount of zeolite A is present in the synthesized products, and they have Si/Al atomic ratios between 1.1 and 1.2.
The above-described problem was eventually overcome when it was discovered that NaLSX with little or no type A zeolite as by-product could be produced by crystallizing the zeolite from a mixed sodium-potassium aqueous gel and then ion-exchanging the zeolite with an aqueous sodium salt, thereby replacing the potassium ions in the product with sodium ions. East German patent 43,221 teaches the production of sodium-potassium zeolite X (Na,K-LSX) by crystallizing a reaction mixture prepared from a sodium aluminate-sodium silicate-sodium hydroxide-potassium hydroxide solution at 50-100xc2x0 C. for about 7-10 hours. British Pat. No. 1,580,928 describes a procedure for the preparation of Na,K-LSX zeolite wherein aluminosilicate gels are subjected to a multi-day aging period at low temperature, ideally 40xc2x0 C., followed by crystallization at 60-100xc2x0 C. at ambient pressure. The principal characteristics of this procedure are that a lengthy aging period is required and the product has significant amounts of potassium ion, the latter characteristic resulting because the process is carried out at a K2O/(K2O+N2O) molar ratio in the range of between 0.10 and 0.40. Accordingly, when it is desired to produce substantially potassium-free NaLSX by this procedure, it is necessary to conduct extensive extra ion exchange steps with sodium salt to replace potassium ion in the as-synthesized powder.
There is a need for an efficient procedure for producing a very low type A zeolite-containing NaLSX by direct crystallization of sodium ion-containing gels. The present invention provides such a procedure.
According to a first broad embodiment, this invention comprises a method of producing sodium X zeolite comprising the steps:
(a) forming a uniform, substantially potassium ion-free, aqueous reaction mixture comprising silica, alumina and sodium ions, the concentrations of the components in the reaction mixture being such that the silica/alumina molar ratio is in the range of about 2.1:1 to about 3.0:1; the sodium oxide/silica molar ratio is in the range of about 1.2:1 to about 2.5:1; and the water/sodium oxide molar ratio is greater than about 35:1;
(b) maintaining the reaction mixture at a temperature less than about 70xc2x0 C. until crystallization of the mixture is substantially complete; and
(c) recovering from the reaction mixture crystallized sodium X zeolite having a silicon/aluminum atomic ratio in the range of about 1 to less than about 1.2.
In a preferred aspect of the first embodiment, the reaction mixture is maintained at a temperature in the range of about 50 to less than about 70xc2x0 C. during step (b). In a more preferred aspect, the reaction mixture is maintained at a temperature in the range of about 50 to about 65xc2x0 C. during step (b).
In another preferred aspect of this embodiment, the silica/alumina molar ratio in the reaction mixture is in the range of about 2.25:1 to about 2.4:1, the sodium oxide/silica molar ratio is in the range of about 1.6:1 to about 1.9:1 and the water/sodium oxide molar ratio is at least about 60:1.
In another preferred aspect of this embodiment, the crystallized sodium X zeolite has a Si/Al atomic ratio in the range of about 1 to about 1.1. In a more preferred aspect of the first embodiment, the crystallized sodium X zeolite has a Si/Al atomic ratio in the range of about 1.03 to about 1.06.
In another preferred aspect of this embodiment the reaction mixture is formed by mixing a first aqueous mixture comprising an alumina source and a second aqueous mixture comprising a silica source. In a more preferred aspect, the first aqueous mixture is an aqueous solution comprising sodium aluminate and the second aqueous mixture is an aqueous solution comprising sodium silicate, sodium metasilicate or mixtures thereof. In a more preferred aspect the first aqueous solution is formed by dispersing sodium aluminate in water and adding sodium hydroxide to the resulting slurry.
In another preferred aspect, the aqueous reaction mixture is vigorously agitated during step (b). In another preferred aspect, the method further comprises drying the recovered crystallized sodium X zeolite at a temperature in the range of about ambient temperature to about 150xc2x0 C.
In another preferred aspect of the first embodiment, the method further comprises ion-exchanging the crystallized sodium X zeolite with monovalent cations, divalent cations, trivalent cations or mixtures thereof. In a more preferred aspect the crystallized sodium X zeolite is ion exchanged with calcium ions, lithium ions, rare earth cations or mixtures thereof.
A second broad embodiment of the invention comprises the crystallized sodium X zeolite product made by a process of the first embodiment. A preferred aspect of this embodiment comprises the crystallized type X zeolite product made by the process of the first embodiment and ion-exchanged with calcium ions, potassium ions, lithium ions, rare earth cations or mixtures thereof.
A third broad embodiment of the invention comprises a method of purifying a gas comprising subjecting the gas to a cyclic temperature swing adsorption process comprising an adsorption process and an adsorbent regeneration step, the crystallized sodium X zeolite prepared in the first broad embodiment being used as the adsorbent in the process.
In a preferred aspect of the third embodiment, the gas being purified is air containing carbon dioxide and the adsorbent adsorbs carbon dioxide from the air.
In another preferred aspect of this embodiment, the crystallized sodium X zeolite has a Si/Al atomic ratio in the range of about 1 to about 1.1. In a more preferred aspect, the crystallized sodium X zeolite has a Si/Al atomic ratio in the range of about 1.03 to about 1.06.
In another preferred aspect of the third embodiment, the adsorbent regeneration step is carried out at a temperature in the range of about 100 to about 250xc2x0 C.
A fourth embodiment is the separation of nitrogen from a gas comprising subjecting the gas to a cyclic adsorption process comprising an adsorption step and an adsorbent regeneration step, using as the adsorbent the crystallized type X zeolite product made by the process of the first embodiment and ion-exchanged with calcium ions, lithium ions, rare earth cations or mixtures thereof. Preferably, the gas is air.
In a preferred aspect of the invention, at least 80% by weight of the crystallized sodium X zeolite has a primary particle dimension in the range of about 0.1 to about 15 microns. In a more preferred aspect, this primary particle dimension is in the range of about 0.1 to about 6 microns.